1. Field of the Invention
This invention relates to detection of leaks in double-wall tanks, pipelines and hulls.
2. Description of the Prior Art
Due to public sensitivity concerning the leakage of dangerous chemicals and the like into the environment, tanks, piping and ship hulls and the like are increasingly provided in double-walled configuration. For example, as a result of the Exxon Valdez oil spill, it is widely anticipated that shortly oil tankers will be required to have an inner hull, containing the oil, and an outer hull spaced from the inner hull for safety. Increasing numbers of states are similarly requiring underground and above ground storage tanks to be double-walled. Pipelines are also commonly double-walled.
However, providing a double-walled pipe, tank or hull is only a partial solution to the problem of leakage. Equipment for detecting leakage of the fluid being contained into the interstitial space between the inner and outer walls is also required, together with a method for locating the leak(s). Desirably, such equipment would remotely and automatically monitor a tank (for example) for leakage, so that little or no labor would be required for monitoring. Preferably, such equipment would be inexpensive to manufacture and install, and would be reliable for long periods of installation. Such a system would desirably provide a binary signal responsive to leakage of the material contained in the tank, pipe or hull, rather than an analog signal which would require periodic calibration. Still more particularly, in connection with underground storage tanks or ship hulls it would be desirable to provide a similar sensor for the detection of ground water, condensation, or sea water, for monitoring the condition of the outer tank or hull.
Several types of equipment have been provided and marketed in recent years in response to these problems. For example, In-Situ, Inc. of Laramie, Wyo. markets a line of monitors for detecting petroleum products in wells. See U.S. Pat. No. 4,682,156 to Wainwright . These devices employ a polymer latch member in a spring-loaded or weighted system for detection of hydrocarbons. When a hydrocarbon leaks into the space surrounding the polymer latch, the latch is dissolved. Bias provided by gravity or by a spring then causes a mechanical member to move upwardly, changing the condition of an alarm indicator at the top of the well. It will be apparent to those of skill in the art that such mechanical devices are unduly complex and subject to failure due to corrosion and the like, and that further elaboration is required if remote monitoring is to be accomplished. Further, such a device must be manually reset after an alarm is given.
A so-called "TraceTek 3000" system is marketed by Raychem Corporation of Menlo Park, Calif. This system is stated to comprise cables formed of a contamination resistant fluoropolymer imbedded with leak detecting wires. It is generally understood by the present inventor that a pulsed time-of-flight circuit is used to detect the location of leaks along the cable. The major disadvantage of this system is that the entire cable must be replaced following a leak. Furthermore, when the cable is moist it is not sensitive to product leaks.
A "Soil Sentry Liquid 330 system" sold by Arizona Instrument Company employs an optical probe. A beam of infrared light aimed at the tip of the probe detects the presence or absence of liquid. If dry, the light is reflected from the inside of the tip and is detected by an optically matched phototransistor. However, if the tip is immersed in liquid, light is refracted out of the probe, breaking the circuit. This device as described would appear to require relatively exacting assembly to insure that the beam is aimed properly at the tip of the probe. Moreover this device would not appear to be capable of differentiating water, for example, from gasoline.
Volatile chemical tracers have also been added to materials which are to be monitored such that the tracer can be detected in the interstitial space between the walls of a double-walled tank. Such systems, employing gas chromatography for tracer detection, are offered by Tracer Research Corporation of Tucson, Arizona. Sample collection and analysis in the operation of this system would be highly labor intensive.
Other systems create a vacuum in a tank to be monitored and detect holes by monitoring changes in the pressure in the tank. This would require a high degree of accuracy of the pressure-sensing instrumentation, and would require compensation for pressure changes resulting from temperature fluctuations.